The relevant parts of my LocalSettings.php read as follows:
$wgEnableUploads = true;
$wgUploadSizeWarning = 0;
$wgMaxUploadSize = 33554432;
$wgUseAjax = true;
$wgCheckFileExtensions = false;
$wgStrictFileExtensions = false;
$wgVerifyMimeType = false;
Although this should allow uploading of all file types, I cannot upload PDF or ODF documents with this configuration. After choosing them in Firefox 3.6 as upload, nothing happens, the input box stays empty, no error message.
I have to explicitly add
$wgFileExtensions = 'pdf';
$wgFileExtensions = 'odt';
$wgFileExtensions = 'ods';
$wgFileExtensions = 'odp';
so it works. Looks like a bug to me.